1. Field of the Invention
The present invention concerns a resonator for magnetic resonance applications, the resonator being of the type having a conductor element that extends from a first conductor end to a second conductor end.
2. Description of the Prior Art
Resonators of the above type are generally known, for example, from DE 103 14 215 A1 (corresponding to U.S. Pat. No. 6,943,551 B). During operation of the conductor element, a resonator current oscillates at a resonator frequency in the conductor element from the first conductor end to the second conductor end and back. The conductor element is tuned to the resonance frequency by a corresponding circuit. In DE 103 14 215 A1 the conductor element is an antenna rod of a birdcage resonator that has further antenna rods, ferrules, ring conductors running axially, centrally around the birdcage resonator and connection lines to the ring conductors. The elements cited above can be components of a multi-layer conductor trace foil.
Like many other resonators, resonators for magnetic resonance applications have a conductor element that extends form a first conductor end to a second conductor end. During operation of the conductor element, a resonance current oscillates with a resonance frequency in the conductor element (also like other resonators) from the first conductor end to the second conductor end and back. The resonator current is particularly high when the conductor element is tuned to the resonance frequency.
In magnetic resonance applications the Larmor frequency with which the magnetic resonance system is operated depends on the strength of the basic magnetic field of the magnetic resonance system and on the chemical element whose excited spin should be detected. For hydrogen (which is the most frequent application case), the gyromagnetic ratio is approximately 42.4 MHz/T.
Magnetic resonance systems are typically operated with basic magnetic fields that lie between 0.2 and 1.5 T. More recently times, magnetic resonance systems have become known that exhibit stronger basic magnetic fields, in particular basic magnetic fields of 3 T, in some cases even in particular to 5 T and more. The Larmor frequency of magnetic resonance system correspondingly typically lies between 8.5 MHz and approximately 63.5 MHz, but can even be above this in individual cases.
The Larmor frequency is the frequency to which the resonators must be tuned in magnetic resonance applications. In the ideal case it thus corresponds to the resonance frequency of the resonator.
As is generally known, a conductor element is resonant at a resonance frequency without further measures when its length is half of the wavelength of the resonance frequency. As results from a simple calculation, the length of a λ/2 rod is thus approximately 2.5 m for a magnetic resonance system with a basic magnetic field of 1.5 T. Such lengths are unrealistic for resonators for use in magnetic resonance applications. For example, the rods of whole-body antennas exhibit lengths that normally are approximately 40 cm, and in practice do not exceed 60 cm. Local coils are often substantially smaller. For this reason, for resonators for magnetic resonance applications it is not possible without further measures to achieve tuning to the Larmor frequency by dimensioning of the conductor element. Rather, it is generally typical to couple the conductor ends with one another via a corresponding circuit, and the circuit effects the tuning of the conductor element to the resonance frequency. The present invention assumes resonators fashioned in such a manner.
Even when the conductor elements in resonators for magnetic resonance systems are significantly shorter than half of the wavelength of the resonance current oscillating in the conductor element, the resonance current is nevertheless at radio-frequency. In the case of radio-frequency currents, an effect known as the skin effect occurs: the resonance current no longer flows in the entire cross-section of the conductor element, but rather only in a boundary or border region thereof. The boundary region exhibits a skin depth that is determined by the resonance frequency and the material of which the conductor element is composed. Due to the skin effect, the resonance current thus flows only in a fraction of the cross-section of the conductor element, so the effective resistance of the conductor element increases. Measurements show an increase that is proportional to the square of the resonance frequency.
It is conceivable to reduce the effective resistance of the conductor element via cooling or by the use of a superconducting material. These procedures, however, would involve a significant technical expenditure and moreover represent a safety risk for a patient who is examined in the magnetic resonance system. They are therefore normally not used in practice in magnetic resonance systems.
The use of a radio-frequency braid is also eliminated in practice. Braided conductors reduce the resistance only up to frequencies of a few megahertz, typically 2 to 4 MHz.
Conductor elements are known that are fashioned as multi-layer conductors. Examples of such multi-layer conductors are disclosed in U.S. Pat. Nos. 2,769,148 and 6,148,221. When, in such a case, the individual layers exhibit layer thicknesses that are smaller than the skin depth, the effective resistance at the resonance frequency can be significantly reduced with such conductor elements. The layers can be either concentric relative to one another (known as a Clogston conductor, see U.S. Pat. No. 2,769,148) or planar (see, for example, U.S. Pat. No. 6,148,221). If such conductor elements could be used in resonators for magnetic resonance apparatuses, this would be advantageous. However, the use of multi-layer conductors as conductor elements does not lead to the expected reduction of the effective resistance without further measures.
More precise tests have shown that the problem is that the optimal distribution of the resonance current among the individual layers of the multi-layer conductor after a transition from a solid (non-layered) conductor or an external circuit to the multi-layer conductor ensues only after a length that is greater than the wavelength corresponding to the resonance frequency. As stated in the preceding, since the resonators for magnetic resonance apparatuses exhibit lengths that are distinctly smaller than the wavelength of the resonance frequency, this current distribution does not have the opportunity to occur. Moreover, slight inhomogeneities of the multi-layer conductor lead to a significant reduction of the achievable resistance decrease. The use of multi-layer conductors in resonators in resonators for magnetic resonance applications has conventionally not been thought to be reasonable in practice.